1. Field of the Invention
This invention relates to MHD generation systems and more particularly to method and apparatus for cooling and protecting the inner duct electrode and insulation wall surfaces utilizing transpiration cooling.
2. Description of the Prior Art
The environment created within a magnetohydrodynamic (MHD) generator duct can be described as hostile. Materials, typically including a fuel such as particulate coal, an oxidant such as air and an ionizing conductor or seed such as potassium are reacted in a combustor to create high temperature reaction products, referred to herein as a plasma. The plasma, typically including excess fuel, coal slag and sulfur at a temperature in the rane of 2500.degree. K., is passed over the inner wall surfaces of the generator duct at a velocity in the range of 1000 meters per second.
The duct wall surfaces, which can include electrodes, electrically conducting wall segments and electrically insulating wall segments, when exposed to these hostile conditions, tend to erode, corrode, evaporate or otherwide deteriorate. One prior art response to the deterioration has typically been to utilize high pressure water cooled metal structures to cool the wall surfaces. Ceramic spacers used as electrical insulation are cooled by conduction through intimate contact with the water-cooled metal. Ceramic coatings have also been provided to buffer the metal surfaces from the plasma.
It appears further to have been universally accepted that, in order to avoid excessive erosion of the inner wall surfaces, particularly the electrode walls, the water-cooled wall surface temperatures should be maintained below the freezing point of the seed/slag mixture so that some of this material will freeze onto the surfaces as a protective layer. However, while prior teachings have claimed that this layer protects the metal or ceramic wall surfaces from the hostile plasma, it appears to react with the surfaces by a process which is at least partially electrochemical. To deal with these reactions there is a tendency to further reduce the surface temperatures, through water cooling, which results in severe material limitations, high electrical resistance of the slag layer, and apparently such high electrical resistance at the slag layer-electrode inner face that detrimental arcing and hot spots result. Additionally, this slag layer provides an undesirable path for current leakage across ceramic insulators which results in lower system efficiency and breakdown of the insulators.
If a slag layer is desired as a protective surface, the combustor must provide some slag carryover into the duct to a degree higher than otherwise would be necessary. This excess slag increases the stack gas cleanup requirements to maintain acceptably low seed material loss and particulate emission from the plant and combines with the seed, making seed separation from collected material difficult. The seed-slag combination also forms very hard and tenacious deposits on component surfaces downstream of the duct, such as heat exchanger tubes, thereby decreasing system efficiency and increasing maintenance concerns.
Conductive water cooling provides additional concerns. The material for conducting heat to the water is severely limited, and copper appears to be the only material with adequate thermal diffusivity to prevent localized electrical arcs. Copper, however, dissolves into demineralized water at the operating temperatures. With or without deionization of the water, it is difficult to maintain the water as an electrical non-conductor, which must be maintained to avoid efficiency losses, and it is further difficult to design and fabricate structures and conduit paths for the water, typically at high pressures, which themselves are insulating and thus not a drain on overall system efficiency.
It is therefore desirable to provide an MHD system which overcomes the difficulties associated with water cooling. It is further desirable to alleviate the concerns associated with excessive slag carryover and buildup.